1. Field of the Invention
In general, the present invention relates to an ultrasonic diagnostic apparatus. In particular, the present invention relates to an effective ultrasonic technology applied to reduction of the amount of focus data for forming received ultrasonic beams of ultrasonic probes with center frequencies different from each other and reduction of the amount of focus data (focus data for forming received ultrasonic beams) for forming a focus of ultrasonic waves received by ultrasonic transducers of a broad-frequency-band ultrasonic probe employed in an ultrasonic diagnostic apparatus.
2. Description of the Related Art
In the conventional ultrasonic diagnostic apparatus, ultrasonic waves transmitted by a plurality of ultrasonic transducers to an object are reflected by the object and received by the ultrasonic transducers. Received signals are each amplified and are delayed by a delaying process (beam-forming process) that adjusts wavefronts coming from a focus in order to electrically converge the received signals output by the ultrasonic transducers. The delayed signals are finally added up to form an ultrasonic beam.
In addition, the focus point of ultrasonic waves received by ultrasonic transducers changes with the lapse of time through a number of stages or in a dynamic manner.
If analog processing is carried out on a received signal, a delay time deliberately introduced in the delaying process described above is converted into tap switching data of an analog delay line which is used for determining a tap on the analog delay line.
If digital processing is carried out on a received signal, on the other hand, a delay time with a predetermined length is deliberately introduced after the analog signal has been converted into digital data in an analog-to-digital conversion process. The delay method is determined in dependence upon the beam-forming technique. Typically, the delay time is introduced as follows. Pieces of digital data (digitized received-signals) are stored at a sequence of addresses in a memory and then read out from the memory starting from an address shifted from the head of the address sequence by an offset to result in a delay time .tau..sub.0 corresponding to the offset. In this case, in order to provide a delay time shorter than the sampling period of the analog-to-digital conversion process, delayed digital data is obtained by interpolation of pieces of digital data read out from the memory.
In an ultrasonic diagnoses apparatus of an electronic scan type, for example, about 100 to 300 rasters are required to form a frame. The rasters are each formed by dynamic reception focusing in the transmission direction of the ultrasonic wave. An image of one frame is formed by sequential scanning in the direction.
Thus, if the number of focus steps per raster is a1, the number of ultrasonic transducers is a2 and the number of rasters is a3, as many as a1.times.a2.times.a3 pieces of focus data for forming received ultrasonic beams are required.
By the way, the conventional ultrasonic diagnostic apparatus may employ a plurality of ultrasonic probes which generally have frequencies different from each other. The examiner selects an ultrasonic probe with a frequency that is appropriate for an object to be diagnosed. Typical frequencies of ultrasonic probes are 3.5 MHz, 5 MHz, 7.5 MHz and 10 MHz.
Thus, in the conventional ultrasonic diagnostic apparatus, focus data for forming received ultrasonic beams is stored in a ROM (Read-only Memory) in advance for each ultrasonic probe employed therein. In order to form a focus of ultrasonic waves received by ultrasonic transducers employed in an ultrasonic probe, the ultrasonic probe reads out the focus data from the ROM.
In addition, an ultrasonic probe operating at frequencies over a broad frequency band is being put to use in recent years. An ultrasonic probe operating at frequencies over a broad frequency band is referred to hereafter as a broad-frequency-band ultrasonic probe. Thus, the same broad-frequency-band ultrasonic probe can be used at typically 3.5 MHz, 5 MHz or 7.5 MHz which is selected according to the object being diagnosed. The examiner sets only the center frequency of a broad-frequency-band ultrasonic probe at a desired frequency. Since the operation of a broad-frequency-band ultrasonic probe is not dependent upon the value of the center frequency, it is not necessary to adjust focus data for forming received ultrasonic beams according to the value of the center frequency in an analog beam-forming process.
If a digital beam-forming process is carried out, however, ultrasonic waves received by ultrasonic transducers are needed not only sampling delayed but also produced a delay time shorter than the sampling period. In this case, a variety of techniques such as interpolation need to be devised to produce a delay time shorter than the sampling period of the ADC. For this reason, it is necessary to re-calculate data for producing such a small delay, by which digital received signal data resulting from the analog-to-digital conversion carried out by the ADC is to be delayed, in order to form a focus of ultrasonic waves received by ultrasonic transducers.
As a result of studying the conventional technologies described above, the inventor of the present invention has identified the following problems.
In the conventional ultrasonic diagnostic apparatus employing a plurality of ultrasonic probes, there is no relation among the ultrasonic probes as far as the design and usage conditions of the ultrasonic probes are concerned. Thus, focus data for forming received ultrasonic beams employed in one ultrasonic probe is different from focus data for forming received ultrasonic beams employed in another ultrasonic probe. For this reason, in the conventional ultrasonic diagnostic apparatus, focus data for forming received ultrasonic beams needs to be provided for each ultrasonic probe. By the way, the amount of focus data for forming received ultrasonic beams for each individual ultrasonic probe is large, raising a problem of an extremely large total amount of focus data for forming received ultrasonic beams for all the ultrasonic probes.
As a result, a ROM with a large storage capacity for storing such data is required, giving rise to a problem of an increased cost of the ultrasonic diagnostic apparatus.
In addition, when a new ultrasonic probe with unknown focus data for forming received ultrasonic beams thereof is incorporated in the conventional ultrasonic diagnostic apparatus, it is necessary to calculate the data. If the focus data for forming received ultrasonic beams for the new ultrasonic probe is known, on the other hand, the data can be stored in the ROM. In this case, however, it is necessary to supply the data for finding a focus of ultrasonic waves received by ultrasonic transducers from the ROM to a controller of a beam-forming circuit for receiving such data, raising a problem of a decreased diagnosing efficiency of a physician, a typical examiner utilizing the ultrasonic diagnostic apparatus.
Even in the case of an ultrasonic diagnostic apparatus employing a single broad-frequency-band ultrasonic probe which is capable of operating at a plurality of transmission/reception frequencies, a problem of an extremely large total amount of focus data for forming received ultrasonic beams for all the transmission/reception frequencies is encountered as is the case with an ultrasonic diagnostic apparatus employing a plurality of ultrasonic probes. In addition, there are also raised a number of problems such as the necessity to calculate data required for interpolation to produce a small delay or to supply focus data for forming received ultrasonic beams stored in the ROM to a controller of a beam-forming circuit for receiving such data. As a result, there is also encountered a problem of a decreased diagnosing efficiency of a physician, a typical examiner utilizing the ultrasonic diagnostic apparatus.